This application is based on Japanese Patent Application No. 198241/1998 filed on Jul. 14, 1998 and No. 198242/1998 filed on Jul. 14, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a distance measuring equipment and method for measuring a distance to an object on the basis of a round-trip propagation time for light to travel between the equipment and an object that reflects the light.
2. Description of the Prior Art
A distance to an object can be obtained by measuring a time period from a transmission point of pulsed light to a reception point of the pulsed light reflected by the object, and using the known propagation speed of light. This method for measuring a distance is used in a lot of fields including civil engineering and astronomy. Though an accuracy of the measurement becomes higher if the pulse width becomes closer to zero in principle, the pulse width is restricted in a range above a certain value depending on a responsiveness of the light source and reception sensitivity. In a general distance measuring equipment and method, the pulse width has a value between 50 and 100 microns that can provide a resolution of approximately a several centimeters. The waveform of the pulse has a single crest.
Conventionally, the time point when the quantity of received light increases and reaches a predetermined threshold is regarded as the time point when the light is received (reception time point). Though the simplest method for detecting the reception time point is to compare the quantity of the received light with a single threshold, the measurement error due to variation of the reflectivity of the object may become conspicuous. It is because that the time when the quantity of the reflected light reaches the threshold may be delayed if the reflectivity is low and the amplitude of the quantity of the received light is small compared with the case where the amplitude is large. Therefore, it is proposed to set a plurality of threshold levels and to detect the time point when the quantity of the reflected light reaches the highest threshold level (Japanese Unexamined Patent Publication Hei 5-100026). By this method, the time point within a range that is close to the peak of the waveform of the received signal is determined as the reception time point, so that the measurement error can be reduced.
However, in the conventional method, the threshold level is not always identical to the maximum quantity of received light. Therefore, the shift of the reception time point due to the variation of the amplitude in the received light is not canceled completely. A small interval between the threshold levels is required in order to reduce the measurement error. Thus, a light reception circuit having a lot of comparators is needed. In addition, if the peak portion of the received waveform is not steep or has double peaks, it is difficult to ensure a desired accuracy of the measurement.
The object of the present invention is to provide a distance measuring equipment and method that can perform the measurement with high accuracy regardless of the reflectivity of the object.
According to one preferable embodiment of the present invention, a distance measuring equipment includes a transmitter for transmitting pulsed light to the outside, a receiver for receiving the pulsed lighted reflected by an object in the outside and performing photoelectric conversion, and a measuring portion for measuring a time period from the transmission time point of the pulsed light to the reception time point of the reflected light so as to output data of the distance to the object. The measuring portion periodically samples the electric signal obtained by the photoelectric conversion of the receiver to memorize plural instantaneous values at plural time points and determines the time point at the peak of the quantity variation of the received light on the basis of the memorized plural instantaneous values so as to generate the data of the distance to the object on the basis of the determined time point as the reception point.
The language xe2x80x9cdistance to the objectxe2x80x9d generally means a numeric information having a value corresponding to the distance to the object, and includes data indicating a round-trip propagation time for the pulsed light to travel between the equipment and the object, as well as a one-way propagation time determined from the round-trip propagation time. The language xe2x80x9coutputxe2x80x9d means to provide information that includes signal transmission to another equipment, display on a graphic or numeric display device and information by a sound signal.
Preferably, the electric signal is an integral analog signal of the photoelectric conversion signal from the received pulsed light.
According to another embodiment of the present invention, the transmitter transmits the pulsed light to the object and the receiver simultaneously. The receiver receives the pulsed light from the transmitter and the object so as to perform the photoelectric conversion. The measuring portion generates the data of the distance to the object on the basis of the determined first point as the transmission time point and the determined second point as the reception time point.
According to the other embodiment of the present invention, the measuring portion detects rising and falling edges of a quantity variation of the received light of the pulsed light on the basis of the electric signal, determines the time point at the peak of the quantity variation of the received light, and generates the data of the distance to the object by regarding the determined time point as the reception time point.
The language of xe2x80x9crising edgexe2x80x9d means the time point when the quantity of the received light becomes a predetermined value for the first time, and the language of xe2x80x9cfalling edgexe2x80x9d means the time point when the quantity of the received light becomes a predetermined value for the last time. If the predetermined value is sufficiently small, the rising edge can be substantially regarded as the start time point of the reception of the light, while the falling edge can be substantially regarded as the end time point of the reception of the light.